This application is a U.S. National Phase Application of PCT International Application PCT/JP01/00848.
1. Technical Field
The present invention relates to an optical information processing apparatus for recording, reproducing or erasing information on an optical medium or magneto-optical medium such as an optical disk or an optical card, and more particularly an optical head apparatus which emits laser beams having different wavelengths and a laser driving apparatus used in the optical head apparatus.
2. Background Art
An optical memory technology which uses an optical disk having a pit-shaped pattern as a memory medium having a high density and a large capacity is finding extending application in a digital audio-disk, a video disk, a document file disk and a data file and the like.
By the optical memory technology, information is recorded and reproduced on an optical disk with high accuracy and high reliability by way of a finely condensed light beam. Recording and reproducing operations depend only on an optical system used for the optical memory technology.
Basic functions of an optical head apparatus which is a principal part of the optical system are classified roughly into converging for forming a diffraction limited fine spot, focal point control and tracking control for the above described optical system, and pit signal detection. These functions are realized by combination of various kinds of optical systems and photoelectric conversion detecting systems depending on purposes and applications.
On the other hand, an optical disk having a high density and a large capacity which is referred to as DVD has recently been put to practical use and spotlighted as an information medium which permits processing a large amount of information such as animations. The DVD optical disk adopts a smaller pit size on an information recording surface so as to have a recording density higher than that on a compact disk (hereinafter abbreviated as a CD) which is a conventional optical disk. Accordingly, an optical head apparatus which records and reproduces (information on) the DVD optical disk uses a light source and a converging lens having a wavelength and a numerical aperture (hereinafter abbreviated as NA) for determining a spot diameter which are different from those for the CD. For reference, a wavelength of a light source is approximately 0.78 xcexcm and an NA of a converging lens is 0.45 for the CD, whereas a wavelength of a light source is approximately 0.63 to 0.68 xcexcm and an NA is approximately 0.6 for the DVD optical disk.
For recording and reproducing (information on) two kinds of optical disks of the CD and the DVD optical disk by one optical information processing apparatus, an optical head apparatus having two optical systems is required. Since a CD-R which has recently been used frequently as a derivative form of the CD permitting additional writing uses a reflecting film on a disk which is optimalized to a wavelength of 0.78 xcexcm and does not allow information reproduction at the wavelength for the DVD in particular, an optical information processing apparatus which records and reproduces information not only on DVD and CD but also on the CD-R must have two light sources having wavelengths of 0.78 xcexcm and 0.63 to 0.68 xcexcm.
From demands for compact and thin configuration and a lower cost of the optical head apparatus, on the other hand, an optical system tends to be used commonly for the CD and DVD, and there are adopted, for example, a system which switches only converging lenses between two kinds for the CD and DVD optical disk and a system which uses also a converging lens commonly while mechanically or optically enlarging only an NA for the DVD optical disk and reducing the NA for the CD.
Furthermore, there has recently been developed a technology which integrates also a light source by forming an infrared laser emitting layer and a red laser emitting layer in a chip of semiconductor laser (for example, LD: laser diode). Furthermore, there has been developed a two-wavelength light source which has two kinds of semiconductor laser chips mounted in a single package.
The above described two light sources will hereinafter be referred to collectively as two-wavelength LDs for convenience. Description will now be made of a driving method for the two-wavelength LD.
FIG. 10 is a block diagram of a conventional LD driving circuit, in which reference numeral 1 denotes a semiconductor laser, reference numeral 2 denotes a photodiode which is a photodetector for monitoring an output from the semi conductor laser. Reference numeral 3 denotes a common terminal, reference numeral 4 denotes a variable resistor, reference numeral 5 denotes an operational amplifier and reference numeral 6 denotes an automatic power control circuit (hereinafter abbreviated as APC circuit).
Description will now be made of operations of the LD driving circuit which is configured as described above. In FIG. 10, the common terminal 3 is maintained at a positive potential with a power source apparatus (not shown). When the APC circuit 6 starts operating, a current is supplied to the semiconductor laser 1 to emit rays from the semiconductor laser 1. When the photodiode 2 receives a portion of the rays emitted from the semiconductor laser 1, a current corresponding to an intensity of the emitted rays is supplied from the common terminal 3 to ground by way of the photodiode 2 and the variable resistor 4, thereby generating a voltage across both ends of the variable resistor 4. The voltage of the variable resistor 4 is detected by the operational amplifier 5 and fed back to the APC circuit 6. The APC circuit 6 controls the current supplied to the semiconductor laser 1 so that the voltage of the variable resistor 4 is a predetermined voltage, thereby maintaining an optical output from the semiconductor laser 1 constant. A resistance value of the variable resistor 4 is adjusted so that the optical output from the semiconductor laser 1 has a desired value.
Though the above described conventional example uses an output monitoring photodetector for a semiconductor laser, a two-wavelength LD which has recently been under development uses only an output monitoring photodetector for two light sources in most cases as shown in FIG. 11.
FIG. 11 is a circuit block diagram of the two-wavelength LD, in which reference numeral 1a denotes a first semiconductor laser and reference numeral 1b denotes a second semiconductor laser. Reference numeral 2 denotes a photodiode which is a photodetector for monitoring outputs from the semiconductor lasers. Reference numeral 3 denotes a common terminal. Due to demands for a low cost and a compact configuration of the light source, the circuit uses only one photodetector 2 and only one output terminal of the photodetector 2. Though 1a and 1b are two semiconductor lasers in FIG. 11, these lasers may be separate light emitting layers of a single laser chip.
When the above described LD driving circuit is used as a circuit for driving such a two-wavelength LD as that shown in FIG. 11, the circuit has such a configuration as that shown in FIG. 12.
FIG. 12 is a block diagram of a circuit for driving the two-wavelength LD, in which members identical to those in FIG. 11 are denoted by identical reference numerals with no description in particular. In FIG. 12, reference numerals 4a and 4b denote variable resistors, reference numerals 5a and 5b denote operational amplifiers, reference numerals 6a and 6b denote APC circuits, and reference numerals 7a and 7b denote electronic switches.
Semiconductor laser driving operations in FIG. 12 will be described briefly. For driving first semiconductor laser 1a, the electronic switch 7a is set in a connected condition, whereas the electronic switch 7b is set in a disconnected condition.
When the first semiconductor laser 1a is allowed by the APC circuit 6a to start emitting rays, a current is supplied to a photodetector 2 by way of the variable resistor 4a by functions of the electronic switches 7a and 7b. Subsequently, the first semiconductor laser 1a can be driven at a desired optical output by adjusting a resistance value of the variable resistor 4a as in the conventional LD driving circuit.
For driving the second semiconductor laser 1b, the electronic switch 7a is set in a disconnected condition, whereas the electronic switch 7b is set in a connected condition. Since a current is supplied to the photodetector 2 by way of the variable resistor 4b, the second semiconductor laser can be glowed at a desired optical output by adjusting a resistance value of the variable resistor 4b. For the above described operations, the APC circuits 6a and 6b as well as the electronic switches 7a and 7b are switched automatically and selectively by a control system (not shown).
However, the above described two-wavelength LD driving circuit has a configuration which requires the electronic switches 7a and 7b, thereby posing a problem to enhance a cost.
Furthermore, the above described two-wavelength LD driving circuits also has a problem that the circuit allows voltage values detected with the operational amplifiers 5a and 5b to be changed due to changes of resistance values which are caused by temperature changes in the connected conditions of the electronic switches 7a and 7b, thereby changing the outputs from the semiconductor lasers 1a and 1b. Accordingly, the two-wavelength LD driving circuit poses a problem to enhances costs and deteriorates temperature characteristics of an optical head apparatus and an optical information processing apparatus which use a two-wavelength LD driven by the two-wavelength LD driving circuit.
The present invention has been achieved to solve the problems described above and has an object to provide a laser driving apparatus which is inexpensive and free from deterioration of characteristics as well as an optical head apparatus and an optical information processing apparatus which use the laser driving apparatus.
One aspect of the present invention is a laser driving apparatus comprising:
n laser light sources L1 to Ln (n is a natural number equal to or larger than 2);
a photodetector of detecting rays emitted from said n laser light sources;
n variable resistors VR1 to VRn connected to said photodetector;
voltage detecting means of detecting n kinds of voltages V1 to Vn related to said variable resistors; and
control means of controlling outputs of said laser light sources so that detected corresponding voltage values are predetermined values, and
said variable resistors VR1 to VRn are connected in series to one another,
the object of the voltage detection is one variable resistor or a serial circuit composed of a plurality of variable resistors connected in series, and
the variable resistors or serial circuits always include variable resistors which are different from each other.
Another aspect of the present invention is A laser, driving apparatus comprising:
n laser light sources L1 to Ln (n is a natural number equal to or larger than 2);
a photodetector of detecting rays emitted from said n laser light sources;
n variable resistors VR1 to VRn connected to said photodetector;
voltage detecting means of detecting n kinds of voltages V1 to Vn related to said variable resistors; and
control means of controlling outputs of said laser light sources so that detected corresponding voltage values are predetermined values, and
said variable resistors VR1 to VRn are connected in parallel with one another and
said voltage detecting means detects voltages V1 to Vn of middle potentials of said variable resistors relative to a reference voltage.
Still another aspect of the present invention is a laser driving apparatus comprising:
n laser light sources L1 to Ln (n is a natural number equal to or larger than 2);
a photodetector of detecting rays emitted from said n laser light sources;
n variable resistors VR1 to VRn connected to said photodetector;
voltage detecting means of detecting n kinds of voltages V1 to Vn related to said variable resistors; and
control means of controlling an output of said laser light source so that a detected corresponding voltage value is a predetermined value, and
said variable resistors VR1 to VRn are connected partially in series to one another and connected partially in parallel with one another,
with regard to a group of variable resistors connected in series,
the object of the voltage detection is one variable resistor or a serial circuit composed of a plurality of variable resistors connected in series,
the variable resistors or the serial circuits always include variable resistors different from each other, and
with regard to a group of variable resistors connected in parallel, said voltage detecting means detects voltages of middle potentials of the variable resistors connected in parallel relative to a reference voltage.
Yet still an other aspect of the present invention is the laser driving apparatus wherein
said n is two and said variable resistors are connected in series, and
said voltage detecting means detects a voltage across both ends of each of said variable resistors.
Still yet another aspect of the present invention is the laser driving apparatus,
said n is two and said variable resistors are connected in series, and
said voltage detecting means detects a voltage across both ends of one of said variable resistors, detects a voltage across both ends of a serial circuit composed of said both variable resistors and detects voltages across both ends of the variable resistors respectively utilizing detected voltage of these two detected.
A further aspect of the present invention is the laser driving apparatus,
said n is two and said variable resistors are connected in series, and
said voltage detecting means (1) detects a voltage across both ends of one of said variable resistors which is connected to a reference potential, (2) detects a potential of a terminal of the other variable resistor which is not connected to said one variable resistor relative to said reference potential, and (3) detects a voltage across both ends of said other variable resistor by subtracting said detected voltage across both the ends from said detected potential.
A still further aspect of the present invention is the laser driving: apparatus,
said variable resistors are connected in series and one of the variable resistors is connected to a predetermined reference potential, and
said voltage detecting means detects a potential of a side of said one of the variable resistors which is not connected to the reference potential relative to the reference potential and a potential of the other variable resistor which is not connected to said one of the variable resistors relative to said reference potential.
A yet further aspect of the present invention is an optical head apparatus, comprising:
the laser driving apparatus;
means of condensing rays emitted from said laser driving apparatus onto an optical information medium; and
a photodetector of detecting reflected rays from said optical information medium.
A still yet further aspect of the present invention is an optical information processing apparatus, comprising at least:
a driving mechanism for an optical information medium;
The optical head apparatus;
a focus servo mechanism using a focus error signal and a tracking error signal respectively obtained from said optical head apparatus;
a tacking servo mechanism;
an electric circuit for realizing said servo mechanism; and
a power source.